chapter  22
Pages 22

Hemicelluloses are major components of plant biomass. In hardwoods and agricultural residues, xylose is the principal hemicellulosic sugar. Xylose and other hemicellulosic sugars are recovered from lignocellulose more readily but are fermented with greater difficulty than is glucose. Xylose metabolism employs pathways distinctly different from those involved in the utilization of glucose. With most yeasts, xylose metabolism requires air. Aeration results in cellular respiration (as opposed to fermentation) and low ethanol yields. It is possible, however, to suppress respiration by feeding small amounts of glucose during the xylose fermentation. Some yeasts, such as Pachysolen tannophilus, will metabolize xylose anaerobically. Alternately, other yeasts will anaerobically ferment the keto isomer of xylose, xylulose, after it is formed from xylose by the action of xylose isomerase. In both instances, the fermentation rates are low. Improved strains of P. tannophilus have been obtained by UV mutagenesis followed by enrichment for faster growth in nitrate-xylitol broth or by selecting for yeast strains incapable of using ethanol as a carbon source. Several yeasts have been described as superior xylose fermenters, including (in approximate ascending order) : Candida troplcalis. Kluyveromyces marxianus. P. tannophilus, the mutant Candida sp. XF 217, and Candida shehatae (and its sexually perfect form, Pichia stipitis) . The xylose fermentation rate of C. shehatae is 3 to 5 times higher than that obtained with P. tannophilus, but the yields of ethanol from xylose are similar with the two organisms. The glucose fermentation rate and ethanol yield are lower with C. shehatae) than with P. tannophilus. Unstable petite and grande strains of C. shehatae have been obtained on urea+xylitol agar, and some show markedly different fermentation rates and products. Further strain improvement and process development should soon provide commercially practicable technology for the fermentation of xylose.